This is a perfect example of clause made largely obsolete over time. Because of travel and communication issues of the late 18th century, when Congress went into recess, it would do so for 6 months or even longer. Vacancies actually did occur during the recess and needed to be filled. Now, travel and communication are easy and the longest recess is a few weeks in the summer. These pro forma sessions are a complete sham. These appointments should be a 50/50 yes or no vote and the minority can suck it.